The light-presentation protocol is an essential feature of circadian rhythm research. The day-night cycle entrains self-sustaining internal pacemakers whose periods may differ from 24 hours. A major problem in the laboratory has been the arbitrariness of illumination regimens. Sudden transitions between darkness and bright light, for example, can disrupt normal circadian adjustment by "shocking" lights and denial of the natural entrainment signal. We are developing an algorithm for naturalistic simulation of illumination changes at specifiable latitudes, day of year, and cloud factor. The project will refine the algorithm and apply it toward the design of an Illuminator mechanism for general laboratory use. The device will simulate outdoor conditions (starlight, moonlight, twilight, and daylight) within a range of 0 to 3000 lux by means of motor driven vanes and electronic dimming. Behavioral tests will concentrate on circadian food- and water-ingestion patterns in a diurnal (ring dove) and nocturnal (rat) species. Subsequent tests will provide an "escape from light" option, allowing the animal self-controlled exposure to veridical outdoor light levels, in simulation of burrowing, If successful, an expanded version of this apparatus will be proposed for potential use in circadian phototherapy for seasonal-depressive disorders.